A horny layer, which is present in the outermost layer of the skin, has a function of suppressing entering of substances from the outside and water evaporation from the inside of the skin, and, at the same time, holding moisture in the horny layer itself so as to keep softness or smooth appearance of the skin. Lipid called a horny intercellular lipid (hereinafter, referred to as a horny ICL) is present in spaces in the horny cells constituting the horny layer such that the lipid is filled in the spaces between the horny cells.
About 50% of lipid composition of the horny ICL is ceramides, and the remainder includes cholesterol, cholesterol ester, and fatty acid. In general, it is known that the reduction of the horny ICL, in particular, a ceramide causes not-preferable skin states such as rough skin, dry skin, and aged skin, and the horny layer reduced in a performance can be improved by externally supplementing the ceramide as a component for improving the function of the horny layer (Non-patent Literatures 1 and 2).
However, since the ceramide has a strong crystalline property, and a high melting point, it is difficult to stabilize the ceramide during formulation. Therefore, in order to blend stably the ceramide into skin external composition, a surfactant and a large amount of various oil agents, or the like, need to be used together, and as a result, it has not been possible to avoid deterioration of a feeling upon use and limitation of the nature of formulation.
Patent Literature 1 describes stabilization of the horny ICL containing a ceramide by using, for example, phospholipid and synthesized surfactants, but use of synthesized surfactants poses problems in terms of safety or a feeling upon use. Furthermore, there has been a problem that phospholipid itself is easily oxidized or degraded.
Furthermore, Patent Literature 2 describes that when a ceramide is emulsified with a sphingosine which is a lipid present in a horny ICL and an acid, the ceramide is not crystallized and can be retained stably. However, combination of the sphingosine and the acid shows cationic property, so that anionic substances cannot be coexistent.
On the other hand, an organic acid containing a sterol group is not the lipid which is present in the horny ICL, but has a similar structure to that of cholesterol sulfate or cholesteryl ester, and therefore has good affinity with the horny layer and high permeability.